High Field Asymmetric Waveform Ion Mobility Spectrometry (FAIMS) is a technology that is capable of separating gas-phase ions at atmospheric pressure. In FAIMS, the ions are introduced into an analytical gap across which a radio frequency (rf) waveform, the magnitude of which is referred to as dispersion voltage (DV), is applied such that the ions are alternately subjected to high and low electric fields. The waveform is asymmetric; the high field is typically applied for one time unit followed by an opposite-polarity low field of half the high field component that is applied for twice as long. The field-dependent change in the mobility of the ions causes the ions to drift toward the walls of the analytical gap. Since the dependence of ion mobility on electric field strength is compound specific, this leads to a separation of the different types of ions one from the other, and is referred to as the FAIMS separation or the FAIMS mechanism. In order to transmit an ion of interest through FAIMS, an appropriate direct current compensation voltage (CV) is applied to compensate for the drift of the ion of interest toward the analyzer wall. By varying the CV, different ions are selectably transmitted through the FAIMS device.
Certain types of ionization sources, such as for instance an electrospray ionization source (ESI), are known to produce ions that are highly solvated. When these highly solvated ions are introduced into the FAIMS analytical gap some of the solvent evaporates from around the ions, thereby contaminating the carrier gas that is flowing through the FAIMS. Unfortunately, FAIMS is highly sensitive to moisture as well as contaminants in the gas that is entering the analytical gap. In fact, frequently contaminants or too much water vapor will result in complete loss of signal and failure of the FAIMS to function properly. Since electrospray ionization involves the high-voltage-assisted-atomization of a solvent mixture, the resulting ion plume contains an amount of water and other volatile solvents that is far too high to be tolerated in FAIMS.
In U.S. Pat. No. 6,770,875, the entire contents of which are incorporated herein by reference, Guevremont et al. teach an ESI-FAIMS combination including a separate desolvation chamber that is disposed between the ESI chamber and the ion inlet orifice of the FAIMS. The desolvation chamber includes a gas inlet orifice and a gas outlet orifice, for providing a gas flow along a direction that is approximately transverse to the direction in which the ions travel between the ESI source and the ion inlet orifice of the FAIMS. A first portion of the gas flow, which is referred to as the counter-current of gas, enters the ESI chamber so as to desolvate the ions and carry neutral solvent molecules away from the ion inlet orifice of the FAIMS. The remainder of the gas flow enters the FAIMS via the ion inlet orifice and serves as the carrier gas for transporting ions within the FAIMS analyzer between the ion inlet orifice and an ion outlet orifice thereof. In practice, often a 50:50 He/N2 carrier gas/desolvation gas mixture is provided into the desolvation chamber; the gas mixture performs both the ion desolvation and ion transport functions that are required for optimal FAIMS operation. Unfortunately, using a flow of a mixed gas for desolvating ions as well as for transporting the ions through the FAIMS analytical gap leads to high consumption of the more expensive carrier gas component.
There exists a need for a FAIMS apparatus and method that overcomes at least some of the above-mentioned limitations.